Optical amplifiers have become an essential component in transmission systems and networks to compensate for system losses, particularly in wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) communication systems. In a WDM transmission system, two or more optical data carrying channels, each defined by a different carrier wavelength, are combined onto a common path for transmission to a remote receiver. The carrier wavelengths are sufficiently separated so that they do not overlap in the frequency domain. Typically, in a long-haul optical fiber system, an optical amplifier would amplify the set of wavelength channels simultaneously, usually after traversing distances less than about 120 km.
One class of optical amplifiers is rare-earth doped optical amplifiers, which use rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica fiber core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimal. Also, the erbium-doped fiber amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression. EDFAs are also attractive because they are fiber devices and thus can be easily connected to telecommunications fiber with low loss.
An important consideration in the design of a WDM transmission system is reliability, particularly when the system is not readily accessible for repair, such as in undersea applications. Since the laser pump is the only active component in the amplification system, it is the most likely to degrade or fail. Such failure would render the optical amplifier, and possibly the optical communication system, inoperative. In order to overcome such an event, several techniques have been developed to design optical communication systems capable of limiting the impact of laser pump failure or degradation. For example, redundancy is sometimes used to obviate optical amplifier failures.
Redundancy can be conveniently employed when two or more optical amplifiers are employed in a single location, which is often the case in a typical long-range optical transmission system that includes a pair of unidirectional optical fibers that support optical signals traveling in opposite directions. In such systems each fiber includes an optical amplifier, which are co-located in a common housing known as a repeater. When multiple amplifiers are co-located redundancy can be achieved by sharing pump energy form all the available pumps among all the amplifiers. For example, in U.S. Pat. No. 5,173,957, the output from at least two pump sources are coupled via a 3 dB optical coupler to provide pump energy to each of two optical fiber amplifiers simultaneously. If one of the pump sources fails, the other pump source provides power to each of the optical amplifiers. Thus, failure of one laser pump causes a 50% reduction in the pumping power of each of the two optical amplifiers. Without such pump sharing, a pump failure could lead to catastrophic failure in one amplifier and no failures in the other. As long as some pump energy reaches each amplifier, there will be enough gain to convey the signals to the next optical amplifier. On the other hand, if any given amplifier were to lose all its pump energy, it becomes a lossy medium and attenuates the signals, usually leading to excessive signal-to-noise ratio at the end of the systems.
While the aforementioned pump redundancy arrangement may be satisfactory for some applications, it would be desirable to provide a pump redundancy arrangement with an even greater degree of reliability, particularly in an optical transmission system that employs multiple pairs of optical fibers.